Friday, April 22, 2011

Making the Case for Alien Life

There's no good evidence to date that life exists, or ever has existed, on worlds beyond the Earth — so it might seem odd that the field of science known as astrobiology is booming. Over the past decade or so, hundreds of biologists, geologists, chemists and astronomers have conducted research and attended conferences on astrobiology around the world, and NASA even has an Astrobiology Institute at its Ames Research Center in California.

That's because there's plenty to think about before we actually find alien life. What form, for example, is it likely to take? Where should we be looking, and how? How did life arise on Earth — was it pretty much inevitable, given the right conditions, or was it a one-in-a-billion kind of thing? Are there enough life-friendly planets out there to make the search worthwhile? (See the Hubble telescope's greatest hits.)

Thanks to progress on all of these questions and more, scientists tend to be more confident than ever that life does exist out in the universe — and First Contact, a book by Washington Post reporter and editor Marc Kaufman, is a powerful reminder of why. Take Princeton geologist Tullis Onstott, whose story, along with those of many others, Kaufman tells. In 1996 Onstott ventured deep into a South African gold mine. Tapping into rock a mile (1.6 km) below the surface, he extracted bacteria that were living cheerfully in harsh conditions completely isolated from the rest of the biosphere.

Onstott's discovery is just one of dozens that established the existence of so-called extremophiles, bacteria and other forms of life that thrive in such absurdly hostile places as the hot springs in Yellowstone National Park, superheated water spewing from cracks in the bottom of the sea and environments laced with acid, heavy metals and even radioactive wastes. Life, in short, can deal with a much wider range of conditions than anyone thought — which means a distant planet needn't be a tropical paradise to be habitable. See the world's most influential people in the 2011 TIME 100

Or take Jeffrey Bada, a marine chemist at the Scripps Institution of Oceanography, in La Jolla, Calif. Back in the 1950s, Bada's mentor, Stanley Miller, had probed the origin of life by passing electricity through a vial of organic chemicals to see what came out. Miller's published experiments were flawed. But Bada has re-examined some of Miller's unpublished ones and found intriguing hints that the origin of life may well be the rule on an Earth-like planet rather than the exception. (See the top 10 scientific discoveries of 2010.)

Kaufman also tackles the question of how many such planets are likely to exist. His lead character here is Paul Butler, now at the Carnegie Institution of Washington, who has been involved in scores of planet discoveries since the first so-called extrasolar planet was found in 1995. By Butler's estimation, as few as 5% of sunlike stars may host a habitable planet — but given that there are tens of billions of sunlike stars in the Milky Way alone, that's still a pretty big number. Butler, moreover, is hardly the only, or even the most accomplished, planet hunter in the business, and his estimates don't take into account the most recent discoveries by the Kepler space probe, which is finding planets by the bucketload.

Unfortunately, the most habitable planet found so far — a world known as Gliese 581g, announced by Butler and his colleague Steve Vogt last fall — is now widely believed not to exist after all. False detections are old news in the planet game, though, and Butler and Vogt appropriately noted at the time that their find would have to be confirmed by others before it could be considered rock solid.

The same applies to evidence of alien life, of course. Those claims have been made as well. It happened in 1996, for example, when scientists looked into a rock blasted from Mars to Earth and saw what they believed was evidence of fossilized bacteria, and earlier this year when an online journal announced a similar discovery in a meteorite that fell in the 1800s. In neither case were any real E.T. remains proved to exist. Back in the 1970s, the twin Viking probes landed on Mars and performed on-site tests of the soil, looking for life. Most came back negative, but one, designed by NASA scientist Gilbert Levin, showed suspicious activity. (See listening for aliens: what would E.T. do?)

In the end, Levin's colleagues, including Carl Sagan, decided it was a fluke — but Levin himself still insists it wasn't, and Kaufman is inclined to give him the benefit of the doubt. Kaufman also bends over backward for the folks who say that bacterial remains can be found in meteorites. "Research in the past decade into the worlds of extremophiles, microbes and fossils," he writes, "has proven that what's true today often is overturned tomorrow, and what's rejected today may be accepted tomorrow."

It's hard to fault Kaufman for thinking the glass is not just half full but nearly overflowing. Even if you limit your thinking to life as we know it — life based on the element carbon and dependent on liquid water for survival — the findings Kaufman writes about (and plenty he doesn't get to) all point to the notion that the Milky Way is likely teeming with biology.

And who's to say life as we know it is the only kind? Physicist Paul Davies has argued recently that the search for life is far too parochial, that alternate biologies could exist, literally under our noses, without our being aware of them. Researchers at Harvard's Origin of Life initiative, meanwhile, are considering the possibility of planets dominated by a sulfur cycle rather than the carbon cycle that prevails on our own planet — and are trying to fathom what sort of life could result. That would be life as we don't know it, but the Harvard group is starting to work out how we might detect it.

If they're right, Kaufman's claim that "before the end of this century, and perhaps much sooner than that, scientists will determine that life exists elsewhere in the universe" may actually be an understatement.